There is a need to determine the properties of fluids in a working environment. For example, it would be advantageous to be able to measure certain electrical properties of lubrication oils circulated in operating machines and engines as a basis for determining the remaining useful life of the fluid. Hydrogen-containing and carbon- and/or silicon-containing lubricants are often used in mechanisms and the lubricants are often heated in their working environments to temperatures in excess of 100° C. Such measurements require a fixture comprising a measurement cell and an enclosure through which a representative stream of the oil is circulated from time to time during its working life. The sensor must be capable of fast and accurate determinations of electrical properties of the fluids over an extended time without being degraded by the material.